Experimental Study of Aerodynamic Sound from a Rotating Disk

The sound generated aerodynamically by a relatively smooth disk rotating in its own plane was measured in an anechoic room. The sound field was broad band owing to random, acoustic, dipole sources caused by the turbulent-boundary-layer flow on the disk. The low-frequency part of the spectrum was due to a uniform distribution of pressure dipoles over the disk surface. The separated flow from the disk periphery had a well-defined periodic vortex development and was the cause of the low-frequency sound. The high-frequency part of the spectrum was due to a distribution of pressure dipoles very near the disk periphery confirming Powell's [J. Acoust. Soc. Amer. 32, 982–990 (1960)] prediction of edge noise. When the surface roughness was one-tenth the boundary-layer thickness, it was found that significant high-frequency sound was generated at the point of roughness, indicating that apparent shear-stress dipoles can be important sound sources. The sound-power level was found to be 70.5 Db for the relatively smooth disk.